During a shot peening process, a stream of shot (i.e., particles), travelling at a high velocity, is directed at a workpiece surface. The shot is directed at the workpiece so as to cause plastic deformation of the workpiece surface, which often is a metal surface. Although the process may be applied for other purposes, the shot peening process generally is used to increase fatigue strength of the workpiece.
For example, residual stress near the surface of high performance machine parts is directly related to fatigue life of the part. If the surface is in a state of residual compression, the growth of microcracks created by, for example, hard processing, will be inhibited. Shot peening is a very effective means for producing surface compression residual stress, and therefore, prolongs the useful life of the part.
Determining the state of surface compression due to shot peening, however, has proven to be very difficult. Until now, no nondestructive method is known for directly measuring the surface compression resulting from a shot peening operation. Specifically, the only known direct method for determining surface compression due to shot peening is to cut the part, and then physically measure the depth, i.e. thickness, of the surface compression. This direct method is time consuming and requires destructing the part being analyzed.
Another known method for determining surface compression due to shot peening generally is referred to as an indirect Almen method. In the Almen method, a strip of material is shot peened, and then the strip is analyzed to determine the surface compression due to the shot peening. The Almen method is indirect in that the effects of shot peening are not measured directly from a workpiece, rather a substitute Almen strip is utilized. The Almen strip method is subject to insensitivity due to process changes which may occur in the peening operations between Almen strip checks. Also, when peening workpieces having contoured surfaces, it is difficult to reproduce the peening conditions on the contour surfaces with a flat Almen strip. Further, variations in the Almen strips themselves render the Almen strip method subject to error.
It is therefore an object of the present invention to provide a nondestructive method and system for directly measuring surface compression due to a shot peening operation.
Another object of the present invention is to provide a method and system for measuring shot peening surface compression which facilitate continuously monitoring surface compression during a peening operation.
Still another object of the present invention is to provide a method and system for measuring shot peening surface compression which are continuous and sensitive to process changes during the peening process.
Still yet another object of the present invention is to provide a method and system for measuring shot peening surface compression which facilitates accurate measurement of surface compression on flat as well as contoured surfaces.